This invention relates to an electroless copper plating solution used for producing printed wiring boards.
An electroless copper plating solution heretofore known comprises a cupric salt such as cupric sulfate, an alkali-soluble complexing agent for cupric ions such as ethylenediaminetetraacetic acid, a reducing agent such as formaldehyde and a pH adjustor such as an alkali hydroxide. But there are problems in that such an electroless copper plating solution is poor in stability of the solution and generally provides a brittle plated film. In order to solve such problems, there are proposed to add various additives such as cyanogen compounds e.g., sodium cyanide, lactonitrile, etc.; nitrogen-containing organic compounds, e.g. .alpha.,.alpha.'-dipyridyl, ethylaminoethanolamine, rhodanine, etc.; and sulfur-containing compounds, e.g., thiourea, benzothiazole, 2-mercaptobenzothiazole, potassium sulfide, etc. (Japanese Patent Unexamined Publication No. 52-1733, Japanese Patent Examined Publication No. 43-12966).
But a plating solution containing an inorganic cyanide such as sodium cyanide, or lactonitrile is poor in adhesiveness to a substrate having through-holes and often brings about semi-spherical blisters on inner walls of through-holes due to stress from plating deposition. There is a tendency to increase blisters with accumulation of by-produced materials in the plating solution. Such blisters easily bring about peeling during the production step, resulting in producing plating voids.
On the other hand, nitrogen-containing organic compounds and sulfur compounds such as thiourea, rhodanine, potassium sulfide, etc. are effective for stabilizing the plating solution, but suppress the deposition rate and give poor surface appearance of deposited copper. Further, the deposited copper obtained by using a plating solution containing such an additive is poor in surface gloss compared with the case of using an inorganic cyanide, and is easily oxidizable since the surface of deposited copper is activated. The adhesiveness between the plated film and substrate is not a problem in a subtractive process wherein a primary electric copper plating is conducted. According to a primary panel electric copper plating-omitting process, copper is deposited in 2-3 .mu.m thick only by electroless copper plating in order to simplify the process, followed by resist formation and copper plating of pattern. When a dry film is directly laminated without chemical or mechanical polishing in such a process, there is a problem of causing a phenomenon of penetration of solder plating under a floating resist due to poor adhesive strength between deposited copper by plating and the resist (dry film) (hereinafter referred to as "underplating").
Further, in the case of an electroless copper plating solution suitable for producing printed wiring boards by an additive process wherein printed wiring boards are produced by only electroless copper plating, there are problems in that mechanical properties of plated films are insufficient, copper films are broken by expansion and shrinkage of printed wiring boards.